On the scattering of high-cycle fatigue life of laser directed energy deposition repaired Inconel 718 superalloy

Due to the cross-regional heterogeneous microstructure in laser directed energy deposition (LDED) repaired Inconel 718 components, their high-cycle fatigue (HCF) life exhibits significant scatter, the origins of which remain largely unexplored. In this work, three distinctive heat treatment schedules, namely double aging (DA), homogenization→DA (HDA) and homogenization→solution→DA (HSDA), are carefully designed to tailor the microstructure of LDED repaired IN 718 alloys. Based on the experimental analysis and crystal plasticity finite element simulations, the effects of phase structures on the HCF behaviors were separated and discussed in detail. The results indicate that the HCF properties of the repaired samples are dominated by the resistant of fatigue in the deposition region. Specifically, for the DA sample, the remained Laves phases within the deposition region deteriorate the HCF property, while their regular distribution results in stable but short HCF life(confidence coefficient, R² = 0.73). However, for the HSDA variants, although the detrimental Laves phases are dissolved, the δ phases with distinct orientations either intensify or alleviate the level of stress concentration, leading to the significant scattering of the HCF life (R² = 0.56). Furthermore, since HDA dissolves Laves phases and avoids the double-edged sword effect of δ phases, the HDA counterparts exhibit the best HCF properties, including the largest fatigue strength (375 MPa) and the most stable HCF life (R² = 0.77). The results obtained in this work provide new theoretical insights into the further improvement of the HCF life of repaired Inconel 718 alloys.